462 



The slender silver tube not only admits ether to the cylinder, but 

 permits air to pass through the four minute holes in streams of small 

 bubbles, which, as in "Regnault's Hygrometer, keep the ether in conti- 

 nued agitation, and therefore in an uniform temperature. The air vapo- 

 rizes ether at each hole ; both are drawn off by the exhaustion when the 

 syringe is made to act, and cold is produced sufficient at all seasons to 

 cause the deposition of atmospheric moisture, or even to freeze it on the 

 silver cylinder, 



It might be supposed, perhaps, that one aperture at the bottom of 

 the cylinder, as in Regnault's instrument, might supply air in such a 

 manner as to produce equalization of temperature. I found it other- 

 wise. Cold is chiefly generated at the point where air is transmitted ; 

 the abstraction of heat takes place chiefly at the expense of the silver, 

 and the ether is cooled as a secondary process. Accordingly, when I 

 employed a cylinder with one opening in the bottom to admit air, the 

 cloud appeared on the bottom long before it could be discovered on 

 any other part ; but when I procured a cylinder perforated with three 

 additional holes, the cloud appeared on all parts at once. The holes 

 must be made by means of the finest sewing needle, made into a drill ; 

 should they be larger than such a drill will make, the vacuum will be 

 supplied with air from the uppermost holes alone, and there only will the 

 condensation of vapour take place for some time. 



On closely observing the formation of dew on a condensing Hy- 

 grometer, made by Negretti and Zambra according to the published 

 instructions of M. Regnault, I found that this very result occurred ; the 

 obscuration took place at the bottom of the silver thimble, where the 

 common air permeates the ether in a single stream ; and not until the 

 included thermometer had lowered two degrees more was the whole 

 thimble clouded. It would, therefore, be always a question which, the 

 initial or the final, was the true dew point. 



Hitherto, to avoid confusion, I have described but one Thermometer ; 

 but in Regnault's instrument there is a second, fixed within an inch of 

 the first, and parallel to it. The bulb of this Theromometer is, like the 

 other, enclosed in a silver cylinder ; but it contains no ether. One of 

 its uses is to show, by contrast, when the other becomes dull with con- 

 densed vapour, and also to indicate the temperature of the air at the 

 time of making the experiment. In my modification of the instrument 

 I use a second Thermometer similarly placed with the same objects, 

 but also with an additional one. This second Therometer is merely 

 screened in front by a half silver cylinder — that is, a cylinder divided in 

 the direction of its axis. This half cylinder is fixed to an arm, remov- 

 able to one side when not in use. I employ this second Thermometer 

 for a purpose that adds greatly to the utility of the instrument. The 

 Psychrometer, or wet-bulb Thermometer, is much in use on account not 

 only of giving the dew point by an easy calculation, but of its affording 

 a certain amount of information by mere inspection. My second Ther- 

 mometer acts in the capacity of a Psychrometer. Its long cylindrical 



